Purification and characterization of a cellulase-free, thermostable xylanase from Streptomyces rameus L2001 and its biobleaching effect on wheat straw pulp

A xylanase purified from Streptomyces rameus L2001 and the biobleaching effect on wheat straw pulp was investigated. The extracellular xylanase was purified 13.3-fold by precipitation with 40–60% (NH4)2SO4, DEAE-52 and CM Sepharose Fast Flow ion exchange chromatography. It appeared as a monomeric protein on SDS-PAGE gel and had a molecular mass of approx. 21.1 kDa, with a specific activity of 3236.6 U/mg. The purified xylanase had an optimum pH of 5.3 and was stable over pH 4.3–6.7. The stable optimal temperature of the enzyme was 70 °C. The xylanase was activated by Co2+ by up to 329% of baseline activity. The xylanase was highly specific towards xylan, but did not exhibit other enzyme activity. Apparent Km values of the xylanase for birchwood and beechwood were 5.8 and 5.3 mg mL−1, respectively. The potential application of the xylanase was further evaluated in wheat straw pulp. The amount of reducing sugars released by the xylanase from wheat straw pulp was significantly greater with increasing time. Enzymatic treatment at a charge of 20 U/g dry pulp for 1 h before hypochlorite (3.8%) treatment revealed an increase in brightness index by 2.8% and increase in residual chlorine by 14.5%.